The present invention is directed to a power plant and, more particularly, to a system and method of determining leakage within a steam turbine.
Most steam turbines having opposing high pressure (HP) and intermediate pressure (IP) sections running at a hot reheat temperature in excess of 1050° F. (566° C.) require an external cooling system in order to maintain acceptable first reheat stage stress levels. As a result of an interaction between the cooling system and internal leakages between HP and IP sections, it is difficult to determine an amount steam leaking between the HP and IP sections. More specifically, in operation, a running clearance exists between a shaft interconnecting the HP and IP sections and a packing assembly that provides a seal about the shaft. The running clearance allows high pressure, high temperature steam to leak from the HP section, along the shaft, to the IP section. The high pressure, high temperature steam leakage affects an overall efficiency of the steam turbine. That is, as steam leakage increases, steam turbine performance decreases.
There have been numerous attempts to determine the amount of leakage in order to adjust the running clearance and packing geometry for enhanced steam turbine performance. At present, an inference method is employed to calculate the amount of leakage. The inference test relies upon measuring an effect on an exit portion of the IP section resulting from changes made to parameters at an inlet portion of the HP section. In essence, the inference method measures an indirect parameter in order to determine enthalpy changes in the exit portion of the IP section to estimate the amount of steam leaking along the shaft. Employing an indirect measurement to determine an amount of leakage results in a solution that is, at best, one step above a guess. Determining the amount of leakage will enable engineers to adjust the running clearance and packing geometry between the shaft and the packing assembly to create added efficiencies in steam turbine operation. Without knowing, within some level of certainty, the amount of high temperature, high pressure steam leaking along the shaft, adjusting the running clearance and packing geometry to enhance steam turbine performance will remain a time consuming, high cost, and inexact trial and error process.